Tankless Water Heater Troubleshooting

Quick solutions for common tankless water heater problems

Safety First

If you smell gas, see sparks, or notice water leaks, turn off the unit immediately and call a professional. Some troubleshooting steps require turning off gas or electricity - only proceed if you're comfortable with these safety procedures.

Quick Problem Finder

No Hot Water

Unit not heating at all

Temperature Issues

Too hot, cold, or fluctuating

Flow Problems

Low pressure or reduced flow

Error Codes

Display showing error messages

Strange Noises

Unusual sounds during operation

Other Issues

Leaks, odors, or performance

!No Hot Water at All

Step-by-Step Diagnosis

1Check Power Supply

First, verify the unit is receiving power:

Electric units: Check circuit breakers, look for tripped breakers
Gas units: Verify gas supply valve is open, check pilot light if applicable
Both types: Look for display lights or error codes on the unit

2Check Water Flow

Tankless units need minimum flow to activate:

Open multiple hot water taps to increase flow rate
Check for clogged faucet aerators or low water pressure
Verify main water supply is turned on

3Reset the Unit

Try a complete system reset:

1. Turn off power (flip breaker for electric, or turn off gas valve)
2. Wait 5-10 minutes for complete shutdown
3. Turn power back on and wait for startup sequence
4. Test hot water at multiple fixtures

🌡Temperature Problems

Water Too Hot

Possible Causes:

• Temperature setting too high
• Malfunctioning temperature sensor
• Incoming water temperature higher than expected
• Unit oversized for actual demand

Solutions:

1. Lower temperature setting on control panel
2. Check for proper ventilation around unit
3. Have temperature sensors calibrated
4. Consider installing mixing valve

Water Not Hot Enough

Possible Causes:

• Unit undersized for demand
• Cold incoming water temperature
• Scale buildup reducing efficiency
• Gas pressure too low (gas units)

Solutions:

1. Increase temperature setting
2. Reduce simultaneous hot water use
3. Schedule descaling service
4. Check gas pressure (professional)

Temperature Fluctuations

Common Causes:

Multiple fixtures turning on/off
Dirty or clogged heat exchanger
Faulty flow sensor
Insufficient gas supply (gas units)

Solutions:

Install buffer tank or recirculation system
Schedule professional cleaning
Replace faulty sensors
Upgrade gas line if undersized

💧Flow and Pressure Issues

Low Water Pressure/Flow

Check These First:

1. Clean faucet aerators and showerheads
2. Check water filter (if installed)
3. Verify main water pressure (should be 40+ PSI)
4. Look for partially closed valves

If Problem Persists:

• Scale buildup in heat exchanger
• Clogged inlet water filter
• Undersized plumbing lines
• Municipal water pressure issues

Unit Won't Turn On (Flow Issues)

Flow Rate Requirements:

• Most units need 0.5-0.7 GPM minimum
• Open multiple fixtures to reach threshold
• Remove flow restrictors temporarily
• Check for clogged inlet screen

Flow Sensor Issues:

• Sensor may be dirty or faulty
• Professional cleaning recommended
• Replacement may be needed
• Check sensor wiring connections

⚠Common Error Codes

Rinnai Error Codes

Code 11:

No ignition

Check gas supply, clean igniter, verify venting

Code 12:

Flame failure

Gas pressure issue, dirty burner, blocked vent

Code 61:

Fan motor issue

Combustion fan problem, check for obstructions

Code 99:

Incomplete combustion

Vent blockage, inadequate combustion air

Rheem Error Codes

Code P1:

Ignition failure

Gas supply, igniter, or control board issue

Code A0:

Inlet thermistor

Temperature sensor malfunction

Code C6:

Thermal fuse

Overheating protection activated

Code U2:

Flow rate

Insufficient water flow to activate unit

Error Code General Actions:

1. Note the exact code and any patterns (when it occurs)
2. Try turning unit off and back on
3. Check obvious issues (gas supply, power, water flow)
4. Consult your owner's manual for specific code meanings
5. Contact manufacturer if problem persists

🔊Strange Noises

Loud Banging/Popping

Usually Indicates:

• Scale buildup on heat exchanger
• Water hammer in pipes
• Thermal expansion

Action Required:

Schedule professional descaling immediately. Continued operation may damage unit.

Whistling/Hissing

Possible Causes:

• Partially blocked vent
• High water pressure
• Loose connections

Check:

• Vent termination for obstructions
• Water pressure (should be under 80 PSI)
• All pipe connections

Crackling/Clicking

Normal vs Abnormal:

• Light clicking during startup: Normal
• Continuous crackling: Scale buildup
• Loud clicking: Ignition issues

If Abnormal:

Professional inspection needed for ignition system or descaling.

When to Call a Professional

Immediate Professional Service

Gas smell: Turn off gas, evacuate area, call gas company
Water leaks: Can cause structural damage
Electrical sparks: Fire hazard
Carbon monoxide alarm: Vent system issue

Schedule Service Soon

Persistent error codes: After basic troubleshooting
Performance degradation: Lower efficiency or capacity
Unusual noises: That don't resolve with cleaning
Annual maintenance: Descaling and inspection

Before You Call: Troubleshooting Checklist

Power & Supply

□ Circuit breakers not tripped
□ Gas valve fully open
□ Main water supply on
□ Unit display showing power

Water Flow

□ Multiple fixtures tested
□ Aerators cleaned
□ Adequate water pressure
□ No visible leaks

Basic Maintenance

□ Recent service/cleaning
□ Filters checked/replaced
□ Venting unobstructed
□ Error codes recorded

If you've checked all items above and the problem persists, it's time to call a professional.